Thinking it over, I got an idea:
T1 comes in via the PSTN port. Some of the DID numbers are connected on the first box, where they get one type of prompt.
The other DID numbers are set into the “bypass” table,sent to the other T1 port that normally goes to the PBX.
But instead of going to the PBX, send them with a crosover cable to the second gateway,where they get the other type of prompt.
Only a theory, never done it, but I may work